Fused salt electrolysis cell having anode with tapered well therein



v. J. REILLY FUSED SALT BLECTROLYSIS CELL HAVING ANQDE um TAPERED WELL,THEREIN sued llay 19, 1967 v INVENTOR VICTOR J. REILLY I AGENT UnitedStates Patent Office 3,544,444 Patented Dec. 1, 1970 3,544,444 FUSEDSALT ELECTROLYSIS CELL HAVING ANODE WITH TAPERED WELL THEREIN Victor J.Reilly, Memphis, Tenn., 'assignor to E. I. du

Pont de Nemours and Company, Wilmington, Del., a

corporation of Delaware Filed May 19, 1967, Ser. No. 639,863 Int. Cl.C22d 3/02 US. Cl. 204-243 8 Claims ABSTRACT OF THE DISCLOSURE A fusedsalt electrolysis cell, particularly for the production of metallicsodium, having a vertical anode surrounded by a cathode, which anode isprovided with a tapered well in the top thereof extending longitudinallythrough the center of the anode in the portion thereof opposite theelectrolysis zone, which Well, in conjunction with perforations,preferably vertical slots, provided in the anode wall, assurescirculation of electrolyte from the well into the electrolysis zone. Byhaving the anode Well tapered from its top towards its bottom, i.e.,towards the end of the anode where it is connected with the powersource, the anode resistance in the cell is desirably reduced withoutadversely affecting electrolyte circulation.

BACKGROUND OF THE INVENTION Sodium is produced commercially by theelectrolysis of a fused electrolyte comprising sodium chloride in cellswhich are basically similar in design to the Downs cell described inDowns US. Pat. 1,501,756. Such cells employ bottom-mounted verticalgraphite anodes each surrounded by a metal cathode, the two electrodesthus defining an annular electrolysis zone. A foraminous diaphragm isinserted in the electrolysis zone to separate the electrolysis products.Chlorine produced at the anode is removed from the upper part of thecell through a gas dome positioned above the anode. Sodium produced atthe cathode rises into a collecting manifold from which it is removedthrough a vertical pipe communicating with the exterior of the cell.

A factor affecting the economical operation of Downs type cells is thecell resistance, and it is evident that by keeping the resistance as lowas possible the cell voltage drop and power costs will be reduced. Inorder to reduce the anode resistance, and thereby reduce the overallcell resistance, the anodes are made as large as possible with respectto the other cell parts. However, there is a limit to the size of anodeswhich can be used because of the adverse effect of excessively largeanodes on electrolyte circulation. Good electrolyte circulation,particularly in the electrolysis zone, is essential for economicalperformance of the cell. To facilitate such circulation, it has beencommon practice to employ anodes provided with cylindrical wells intheir centers, which wells communicate with the electrolysis zonethrough slots or holes provided in the anode wall. Such wells and slotspermit circulation of electrolyte through the anode well into theelectrolysis zone, thereby increasing the overall performance of thecell.

The benefits derived from employing anodes provided with wells inconjunction with perforations in the anode wall whereby electrolytecirculation is enhanced, have long been recognized. Combinations of suchanode wells and slots are disclosed in US. Pats. 2,194,443, 2,390,548,2,414,831, 2,755,244 and 2,921,894. As shown in such patents, the Wellin the anode has always been of cylindrical design and in the center ofthe anode. Various types of perforations communicating with such a wellhave been proposed, although vertical slots appear generally to bepreferred.

While the provision of Wells in the anodes, in conjunction with theabove-mentioned slots, is advantageous from the standpoint of improvingcirculation of the electrolyte, use of such wells has the distinctdisadvantage of increasing the anode resistance. In the past, theadvantage in improved circulation has been regarded as offsetting thedisadvantage of increased anode resistance resulting from the provisionof anode wells. Thus, the use of anodes with wells of cylindrical designtherein opposite the electrolysis zone together with slots in the anodewall has become more or less common practice. It has now been found thata significant reduction in the anode resistance, compared with theresistance of similar anodes provided with wells of the previouscylindrical design, can be obtained by employing anodes having a well ofimproved design described below. Furthermore, such reduction in theresistance can be achieved without adversely affecting electrolytecirculation.

SUMMARY OF THE INVENTION The invention relates to a Downs type cell forthe production of alkali metals by the electrolysis of fused alkalimetal halide electrolytes, particularly for the production of sodium andchlorine by the electrolysis of a fused electrolyte comprising sodiumchloride. Such cells comprising a bottom-mounted vertical anode,generally cylindrical in shape, surrounded by a cathode, which isusually supported by side arms extending through the side Walls of thecell, a foraminous diaphragm positioned between the electrodes, andmeans for separately removing the electrolysis products from the upperpart of the cell. Such cells may contain a single pair of electrodes,i.e., an anodecathode pair, or an assembly of several such pairs,generally four. The invention contemplates the use in such a cell of ananode having a tapered well in the top thereof extending longitudinallythrough the center of the anode in that portion thereof opposite theelectrolysis zone, which well is in open communication with theelectrolysis zone through perforations, preferably slots, in the anodewall which permit effective circulation of electrolyte from the wellinto the electrolysis zone. The taper of the anode well is essentiallyfrom its top to its bottom, i.e., towards the end of the anode that isconnected with the power source. Since the anode customarily risesvertically from the bottom of the cell where contact with the powersource is effected, the well will be in the upper portion of the anodeand will taper towards its bottom, i.e., the diameter of the well willbe greater at the top than at the bottom. It has been found that anodeswith such tapered Wells which communicate through perforations in theanode wall with the electrolysis zone to permit the desired circulationof electrolyte exhibit a substantially lower resistance in the cell thando otherwise similar anodes having Wells of the conventional cylindricaldesign. Furthermore, the tapered design of the anode well does notsignificantly reduce electrolyte circulation.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a side view in elevation of ananode having a tapered well therein in accordance with the invention.

FIG. 2 is a plan view of the anode of FIG. 1 shown with a surroundingcathode and an annular diaphragm positioned between the anode and thecathode.

FIG. 3 is a vertical cross-section of the structure of shown, eachextending vertically from the bottom of well 3 and terminating a littleshort of the top of the anode so as to leave a solid collar which isdesirable to impart greater strength but is not essential for thefunctioning of the anode. Anode 1 is mounted at the bottom of the cell(not shown) so as to form intimate contact with a metal conductivemember 8 which is connected with the power source.

Shown surrounding top portion 2 of anode 1 is a cylindrical metalcathode 6 which is generally supported 'by cathode arms (not shown)extending through the cell walls (not shown) where contact with thepower source is made. An annular foramino'us diaphragm 7, usually asteel Wire screen, is positioned in the electrolysis zone, i.e., thezone between cathode 6 and the upper portion 2 of the anode. Diaphragm 7is usually rigidly fastened to a removable product collector assembly(not shown) from which it is suspended in the position indicated in thedrawing. It will be noted that well 3 in the anode is generally oppositethe electrolysis zone and is substantially coextensive in verticallength therewith.

DESCRIPTION OF PREFERRED EMBODIMENTS 'well 3 in the anode will haveadiameter at its top which is about twice its diameter at its bottom,with the diameter at the top being approximately halfthe diameter of theanode. In the preferred embodiment illustrated in the drawing,electrolyte communication between well 3 and the electrolysis zone isprovided by four slots 4 equally spaced radially about the circumferenceof the upper portion 2 of the anode. Depending upon the width of suchslots, more or fewer than four slots, e.g., from 2 to 8 equally spacedabout anode portion 2, may be employed, but usually four positioned asindicated with each having a width equal to from about 5 to 20%,preferably 8 to 12%, of the top diameter of the anode well, aregenerally adequate.

Desirably, tapered Well 3 will be no larger than is necessary toprovide, in conjunction with slots 4, eflicient circulation ofelectrolyte from the well into the electrolysis zone, otherwise, theelectrical resistance of the anode in the cell will be undesirablyincreased. In general, the diameter of the well at its top will be equalto from 30 to 80%, preferably 40 to 60%, of the diameter of the anode.In order to permit a significant reduction in the anode resistance whilestill permitting realization of effective circulation of theelectrolyte, the diameter of the well at its bottom should generally beno greater than 70% of the diameter of the well at its top. In general,the diameter of the well at its bottom will be equal to from 0 to 70% ofthe diameter at its top; most preferably, it will be equal to from 30 to60% of the diameter at the top. Thus, the general shape of the well maybe that of an inverted cone but preferably will be that of an invertedtruncated cone, the latter being generally preferred as indicated.

The perforations in the anode wall providing communication between thewell and the electrolysis zone may be in the form of several holesrunning transversely through the walls of the anode. Such holes, whenemployed, should be distributed more or less uniformly over that portionof the anode which together with the cathode defines the electrolysiszone, e.g., that'portion of the anode which is generally coextensivewith slots 4 in the drawing. Instead of transverse holes, sloping holesmay also be employed. However, the preferred type of perforations arethose in the form of vertical slots such as slots 4 in the drawing. Thenumber thereof and their width should be sufficient to insure thedesired electrolyte circulation and they should be uniformly spacedabout the anode.

With respect to the tapered well in the upper part of the anode, thesides thereof are preferably straight with the taper thereof beingdownward as indicated previously. However, tapered wells with side wallsthat are somewhat curved, either inwardly or outwardly, can be used,although they are not as advantageous as the preferred straight sidedtapered wells. As indicated in the drawing, the tapered well should bein the center of the anode and symmetrical with respect to thelongitudinal axis of the anode, otherwise distribution of the currentflow to the active anode surfaces will not be uniform.

Six Downs type sodium cells, constituting a Test Group, were providedwith anodes of the design indicated in the drawing. Each anode includeda tapered well centered in approximately its upper half so as to beopposite and approximately coextensive in length with the electrolysiszone. The top diameter of the wells was one-half the diameter of theanodes, while the bottom diameter of the wells was one-half their topdiameter. Each anode was also provided with four vertical circulationslots positioned as shown in the drawing, the width of each slot beingabout 20% of the diameter of the bottom of the anode well and about 10%of the diameter of the top of the anode well. Nine other Downs typecells, constituting a Reference Group, were provided with anodes havinganode wells therein which were of the conventional cylindrical designwith a top and bottomdiameter equal to the top diameter of the wells ofthe anodes in the cell of the Test Group. Except for the design of theanode wells in the anodes employed, the cells of the Test Group andthose of the Reference Group were the same.

The cells of each of the above groups were operated to produce sodiumand chlorine over periods of time ranging from about 9.5 to 20.8 monthsand the statistical values for power consumptions (kilowatt-hrs, D.C.)per lbs. of sodium produced (averagelife-to-date) were determined. Theaverage power consumption per 100 lbs. of sodium produced in the TestGroup of cells was 423.4 kilowatt hours, Weighted for the age of thecells, whereas the corresponding value for the Reference Group of cellswas 426.4 kilowatt hours.

The power consumption values reported above correspond to a reduction ofabout 0.06 volt in favor of the cells of the Test Group, i.e., the cellswith tapered anode wells. The lower power consumption in the cells ofthe Test Group resulted from a reduction of the resistance of the anodeshaving tapered wells. The difference in power consumption found issignificant and distinctly advantageous, since it is worth about 1.5cents per 100 pounds of sodium with power at 0.5 cent per kilowatt hour.

Further advantages result from the use of anodes with tapered wells, inthat the voltage on such cells does not rise as rapidly with age, aswith cells with cylindrical-well anodes. This results from the factthat, as the anodes wear, the cross-sectional area of a tapered-wellanode reduces proportionally less than that of a correspondingcylindrical-well anode. Thus, with anodes with dimensions in thepreferred ranges, a new tapered-well anode has about 25% morecross-sectional area than the corresponding cylindrical-well anode nearthe base of the electrolysis zone.

However, near the end of the useful life of such anodes, thetapered-well anode has about 50% more cross-sectional area. This isimportant since power loss in older cells dictates the current load theymay carry, as well as the age at which they must be retired. In terms ofvoltage reduction near the end of the useful life of the cells, thetaperedwell has an advantage of about 0.1-0.15 volt. The net effect ofthe reduced average voltage, and the greater reduction in voltage witholder cells, is an increase in current capacity of about 23%. A giveninvestment in electrolytic cells and associated facilities can therebyproduce 23% more sodium.

I claim:

1. In a fused salt electrolysis cell having a vertically positionedcylindrical anode and a surrounding cylindrical cathode defining anelectrolysis zone therebetween, the improvement wherein said anode has atapered well in the top thereof extending longitudinally through thecenter of said anode in the portion thereof opposite said electrolysiszone, the taper of said well being substantial and being essentiallyuniform from its top to its bottom, which well is in open communicationwith perforations provided in the anode wall for circulation ofelectrolyte from said well into said electrolysis zone.

2. A fused salt electrolysis cell in accordance with claim 1, whereinthe anode is cylindrical and the diameter of the anode well at its topis equal to 30 to 80% of the diameter of the anode, and the diameter ofthe anode well at its bottom is no greater than 70% of the diameter ofsaid well at its top.

3. A fused salt electrolysis cell in accordance with claim 2, whereinthe diameter of the anode well at its top is equal to 40 to 60% of thediameter of the anode, and the diameter of said well at its bottom isequal to 30 to 60% of the diameter of said well at its top.

4. A fused salt electrolysis cell in accordance with claim 2, whereinthe diameter of the anode well at its top is about one-half the diameterof the anode, and the diameter of said well at its bottom is aboutone-half the diameter of said well at its top.

5. A fused salt electrolysis cell in accordance with claim 1, whereinthe perforations in the anode wall are a plurality of vertical slotsequally spaced around the anode With each slot having a Width equal toabout 5 to 20% of the diameter of the anode well at its top.

6. A fused salt electrolysis cell in accordance with claim 2, whereinthe perforations in the anode wall are a plurality of vertical slotsequally spaced around the anode with each slot having a width equal toabout 5 to 20% of the diameter of the anode well at its top.

7. A fused salt electrolysis cell in accordance with claim 3, whereinthe perforations in the anode wall are a plurality of vertical slotsequally spaced around the anode with each slot having a width equal toabout 5 to 20% of the diameter of the anode well at its top.

8. A fused salt electrolysis cell in accordance with claim 4, whereinthe perforations in the anode wall consist of four vertical slotsequally spaced around the anode and generally coextensive in verticallength with the length of the anode well, each of said slots being of awidth equal to about 8 to 12% of the diameter of said anode well at itstop.

References Cited UNITED STATES PATENTS 2,415,494 2/1947 Holden 204284 XR2,490,730 12/ 1949 Dubilier 204272 XR 2,604,441 7/ 1952 Cushing 204280XR JOHN H. MACK, Primary Examiner D. R. VALENTINE, Assistant ExaminerUS. Cl. X. R. 204284, 272

